The present invention relates to a handoff control system in a cellular communication system and, more particularly, to a handoff control system and method which perform handoff processing by assigning priorities.
Recently, the number of subscribers in mobile communication systems is unceasingly on the increase, and hence an increase in subscriber capacity is required. To increase the subscriber capacity of a mobile communication system, a technique of reducing the cell radius has been studied. If the cell radius is reduced, the number of base stations in the service area increases. As a consequence, the number of channels used by subscribers can be increased.
Assume that a terminal 110 is present in a cell 130a of a base station 120a and is receiving service from the base station 120a, and the terminal 110 moves into a cell 130b of a base station 120b, as shown in FIG. 9A. As the terminal 110 moves away from the base station 120a, the reception signal strength from the base station 120a gradually decreases in the terminal 110. In this case, the reception signal strength indicates the magnitude of power received from the base station.
The reception signal strength from the base station 120a is periodically measured in the terminal 110. When the measured reception signal strength becomes equal to or less than a predetermined threshold, the terminal 110 sends a handoff request to the base station 120a. With this operation, the terminal 110 is set in a state in which it can also receive service from the base station 120b. 
The handoff request sent from the terminal 110 to the base station 120a is notified from the base station 120a to the base station 120b through a network (not shown). Thereafter, the terminal 110 can receive service from both the base stations 120a and 120b. When the terminal 110 further moves away from the base station 120a, the service from the base station 120a is stopped, and the terminal 110 receives service from only the base station 120b. 
An area where the terminal 110 can receive service from both the base stations 120a and 102b is an area 140 where the cells 130a and 130b overlap.
When the cell radius is reduced as shown in FIG. 9B, the above handoff operation is frequently performed. For this reason, as the cell radius decreases, the handoff rate increases, and forced termination of communication tends to occur.
A method of avoiding such a situation by preparing a queue for requests from terminals that demand handoffs is disclosed in, for example, D. Hong et al., xe2x80x9cTraffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff proceduresxe2x80x9d, IEEE Trans. Veh. Technol., vol. VT-35, August 1986 (reference 1) and Q. A. Zeng et al., xe2x80x9cPerformance analysis of mobile cellular radio system with priority reservation handoff proceduresxe2x80x9d, IEEE Proc. VTC-94, vol. 3, June 1994 (reference 2).
According to reference 1, of all the set channels, some number of channels are always ensured as handoff channels, and the handoff channels are not used for new calls. With this setting, the loss probability during handoff operation is reduced.
According to reference 2, a buffer for new calls is used in addition to the technique disclosed in reference 1 to decrease the loss probability of new calls without increasing the blocking probability and probability of forced termination of handoff calls much.
In addition, Japanese Patent Laid-Open No. 7-264656 (reference 3) discloses a technique of assigning priorities to handoff processes on the basis of a mathematical expression set in consideration of the moving speeds and directions of terminals, and performing the handoff processes in accordance with the priorities.
Terminals move across cells at various speeds. For example, the moving speed of a terminal that moves on a car differs from that of a terminal carried by a walking user. When the moving speeds of terminals differ in this manner, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes. For this reason, when handoff processes are performed in the order of handoff requests, forced termination of communication may occur at a terminal that is moving at a high speed if the time between the instant at which a handoff request is generated and the instant at which the handoff process is completed is prolonged.
In addition, a terminal that moves at a high speed generates handoff requests many times during one communication operation because it passes many cells in a predetermined period of time, and hence the frequency of forced termination of communication tends to increase.
Furthermore, terminals move across cells through various routes; some terminals move away from the base stations from which service is received, and some terminals move while keeping distances to the base stations constant.
When terminals move through different routes as described above, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes as well. When handoff processes are to be simply performed in the order of handoff requests, a delay in performing a handoff process upon generation of a handoff request may cause forced termination of communication at a terminal that only moves away from the base station from which service is currently received because of a process delay.
In the technique disclosed in reference 3, since complicated arithmetic operation is required to assign priorities, and a priority is assigned to each call, processing for a handoff request is frequently performed and complicated.
It is an object of the present invention to provide a handoff control system and method which can decrease the probability of forced termination of communication by considering the time allowed between the instant at which a terminal generates a handoff request and the instant at which the handoff process is completed.
In order to achieve the above object, according to the present invention, there is provided a handoff control system for performing handoff processing for a terminal that moves across cells of a plurality of base stations while performing speech communication, comprising measuring means for periodically measuring a reception signal strength in the terminal, calculation means for calculating the relative change amount of reception signal strength at measurement time intervals on the basis of the measurement result output from the measuring means, queue storage means storing, in advance, a plurality of queues to which priories based on relative change amounts of reception signal strength are assigned, request processing means for, when the terminal generates a handoff request, distributing a call from the terminal to one of the queues on the basis of the relative change amount of reception signal strength in the terminal, and queue control means for, when an available channel is present in an adjacent cell to which the terminal, which generated the request, has moved, assigning the call, distributed into the queue according to the order based on the priority, to the available channel.